The present invention relates to an air/fuel ratio control system for an internal combustion engine, and, in particular, relates to an air/fuel ratio control system for an internal combustion engine which is supplied with air-fuel mixture by a carburetor.
For purifying the exhaust gases emitted by an internal combustion engine of noxious components contained therein, such as HC, CO, NOx, simultaneously, there is known a 3-way catalytic converter, which is capable of reducing the amounts of unburnt hydrocarbons, and also the amounts of nitrous oxides, simultaneously. However, for an engine provided with such a 3-way catalytic converter, in order for the 3-way operation of the convert to proceed properly, it is necessary to control the air/fuel ratio of the exhaust gases of the engine which are passing into the catalytic converter, i.e. the so-called "secondary" air/fuel ratio, within a rather narrow range near to stoichiometric, in order effectively to utilize the 3-way catalytic converter to purify the exhaust gases to an acceptable level of all of the abovementioned undesirable components.
In order to perform this control of secondary air/fuel ratio, in an internal combustion engine which is equipped with a 3-way catalytic converter, the primary air/fuel ratio, i.e. the air/fuel ratio of the air-fuel mixture which is supplied by the carburetor to the engine, is conventionally set to be slightly lower than stoichiometric, i.e. to be on the rich side of stoichiometric, and then a certain amount of secondary air is supplied into the exhaust manifold of the internal combustion engine, before the 3-way catalytic converter. The oxygen in the exhaust gases is continuously detected with an oxygen detector, such as an O.sub.2 sensor, or the like. The abovementioned supply of secondary air to the exhaust manifold is controlled, according to the signal from the oxygen detector, in such a way that the secondary air/fuel ratio of the exhaust gases, before they are introduced into the 3-way catalytic converter, is controlled within said certain narrow range around stoichiometric. Thereby, the operation of the 3-way catalytic converter is effectively assured, both with regard to reduction of the amount of unburnt hydrocarbons in the exhaust gases, and also with regard to reduction of the amount of nitrogen oxides.
However, a disadvantage exists with this prior art method as described above, in that it is necessary for the primary air/fuel ratio of the air-fuel mixture supplied by the carburetor to the internal combustion engine to be substantially smaller, i.e. for the air-fuel mixture to be substantially richer, than stoichiometric, in order to ensure that this level can be raised to stoichiometric by addition of secondary air after combustion of the air-fuel mixture in the combustion chambers of the engine, and, accordingly, the fuel economy of such an engine is not extremely good. Further, the amount of uncombusted hydrocarbons in the exhaust gases within the exhaust manifold, before the catalytic converter, will be rather increased, and, although by the addition of secondary air the overall air/fuel ratio of these exhaust gases is brought close to stoichiometric, nevertheless the absolute amount of uncombusted hydrocaarbons in the exhaust gases, along with the amount therein of oxygen corresponding thereto, is rather high, and the processing of these by the catalytic converter, although it can be performed satisfactorily, may well cause the converter to become rather overheated.
In consideration of the above described problems, it has been previously thought of, and proposed, to provide an air/fuel ratio control device which keeps the primary air/fuel ratio within a certain narrow range near stoichiometric, in order to allow for effective utilization of a 3-way catalytic converter, by controlling an amount of air bled into a fuel supply which is passing through a fuel passage of the carburetor to be supplied to the air intake passage of the engine, according to the signal from such an oxygen detector which is detecting the amount of oxygen in the exhaust gases emitted from the engine. The intended object of such an air/fuel ratio control device as described above, which controls the primary air/fuel ratio of the internal combustion engine, is satisfactorily attained, but a disadvantage arises in that the internal combustion engine is always operated at an air/fuel ratio which is very close to stoichiometric, and, accordingly, in an operating condition for which a rich mixture is required for the internal combustion engine, with a smaller air/fuel ratio than stoichiometric, such as a high load operating condition, or a cold engine operating condition, the performance of the engine is significantly worsened, due to the aforesaid maintenance of the primary air/fuel ratio within a narrow range around stoichiometric. This severely reduces the drivability of a vehicle incorporating such an engine.
In such a case, the drivability of the vehicle, and the usability of the engine, will not be damaged, if the primary air/fuel ratio is corrected to be richer than stoichiometric, when the particular operating conditions of the internal combustion engine demand this; but, if this is done, then accordingly the secondary air/fuel ratio will be smaller, i.e. the exhaust gases will be richer, than stoichiometric, and accordingly the 3-way catalytic converter will not effectively perform its 3-way function; in fact, the catalytic converter will effectively reduce the amount of nitrogen oxides present in the exhaust gases, but will not effectively reduce the amount of unburnt hydrocarbons present therein.